In recent years, solar cells have been greatly interested as a clean energy source, and in particular, silicon solar cells that are highly efficient in power generation have been actively researched for improvement in conversion efficiency and cost reduction as the most promising candidate for power supply targeting the high end market, such as housing.
In such a situation, in order to improve the conversion efficiency by transmitting more sunlight, a method is known in which an antireflective film having a uniform film thickness on the entire surface of a transparent member of a solar cell module is formed to reduce the reflectance utilizing a difference in the refractive index from the transparent member.
As one of the methods of forming such film, a method of forming a metal oxide, called as a sol-gel method, is proposed in which metal alkoxide and an organic solvent are mixed and hydrolysis is carried out as follows using water and a catalystSi(OR)4+4H2O→Si(OH)4+4ROH,thereby producing a hydroxide to condense the reactants as follows (for example, refer to Japanese Patent Laid-Open No. 2003-201443)Si(OH)4→SiO2+2H2O.
An antireflective film formed by the sol-gel method is a porous material having silica particles and a matrix retaining the silica particles. Voids in the film have a refractive index substantially the same as that of an air (refractive index of 1.0), so that even when the microparticle material and the matrix retaining the particles have a large refractive index, the entire film has a refractive index close to that of an air as a result. Accordingly, forming the film on a transparent member enables to reduce the reflectance.
A solar cell module has a plurality of photovoltaic cells that are connected electrically and are placed at predetermined intervals from each other. Incident light in each photovoltaic cell contributes to the conversion efficiency by conversion to electricity while incident light in a clearance between the photovoltaic cells is not converted to electricity and does not contribute to the conversion efficiency. Accordingly, it is essential, for improvement in conversion efficiency, to efficiently introduce incident light from a light receiving surface of a solar cell module to each photovoltaic cell, not to a clearance between the photovoltaic cells.
As a method of solving the above problems, shown in FIGS. 5 and 6 are examples of, not forming the antireflective film 4 having a predetermined film thickness on the entire surface of a transparent member 5 of a solar cell module, but forming an antireflective film 4 having a predetermined film thickness only in a region immediately above each photovoltaic cell 1 and reducing the film thickness of the antireflective film 4 in the regions other than the regions immediately above the photovoltaic cells 1 thinner than that in the regions immediately above the photovoltaic cells 1 (refer to FIG. 5) or to zero (refer to FIG. 6), as disclosed in Japanese Patent Laid-Open No. 2003-152202.